I'm on lithium, 2 weeks ago i had pink spotting for an hour, than a 2 days later the same thing and a pg test came back negative what is going on?

Not likely related: Your spotting may be unrelated to your lithium use. However you need to further investigate its cause as regular pregnancy test may not yet be positive depending on what stage you are in your menstrual cycle. Lithium has been associated with cardiac abnormalities in offsprings of patients on the medication hence the need for caution with its use especially in the early stages of pregnancy.